In all of my languages, the real progress has been by myself. But people usually don't believe it, when I say that classes or teachers, that I at times happened to enounter, were more of an obstacle than help.
But there is one language, where your question applies. My Italian. I don't think that one unfortunate tutoring lesson (mainly filled with the teachers comments of two types: "oh, how is it possible that you actually speak that well after self teaching" and "no, all this is wrong, you'll need to do as I tell you, change the plan completely, slow down, and surely not reach your goals in time").Ask me again in November, when I'll have taken the CELI C1. I hope to have a solid answer then
So far, my "by myself" language achievements were these:
-from very weak B2 to C2 French.Before that there were several "all by myself" phases, mostly focused on heroic staying the same level in spite of being sabotaged by school.
-from forgotten and rusty A1 Spanish to solid active B1 and passive B2/C1 Spanish in two months
-from 0 to A1 German in a month or so. Then there was an unfortunate not too useful class in country (that disqualifies me from "all by myself questions"), some years of not really progressing, and then from rusty A1 to certified B2 in five months on my own.
-from being the worst at English out of all the students in my year (+destroyed confidence, anxiety attacks connected to English, etc), to getting into the top 1/8 of the year (so, among the best 15 out of 120) over one summer by myself.
But the first ALL BY MYSELF, that nobody can question, has been Italian. I've gotten to the point of speaking quite ok, very high comprehension. I'd say B1 speaking and C1 comprehension nowadays. But I'll be able to give a very clear and undenyable and undoubtable answer only after the C1 exam in November.